Protection for Fibre Optic Access Networks

ABSTRACT

An Optical Network Unit, ONU, and an Optical Line Terminal, OLT, as well as a respective method therein, are provided for enabling the ONU to communicate with the OLT. The ONU and at least one OLT are comprised in a passive fibre optic access network. The passive fibre optic access network comprises two fibre paths through the network, a main fibre path and a back-up fibre path, connecting the ONU with the OLT. When a fault is detected in the main fibre path is detected, transmission rate negotiation is performed between the ONU and the OLT over the back-up path. When a transmission rate is agreed upon between the ONU and the OLT during said transmission rate negotiation with regard to communication over the back-up path, the communication between the ONU and the OLT is executed using the agreed transmission rate over the back-up path.

TECHNICAL FIELD

Embodiments herein relate generally to protection for fibre optic accessnetworks, and in particular to protection of connection between anOptical Network Unit and an Optical Line Terminal in a fibre opticaccess network.

BACKGROUND

Fibre optic access networks connect users to a switching equipment of anetwork operator for processing and further transport of data signalstoward a core part of the network.

The access network can be either point-to-point (e.g. Ethernet P2P) orpoint-to-multipoint fibre topology (e.g. Ethernet Passive OpticalNetwork, EPON, Gigabit Passive Optical Network, GPON, or WavelengthDivision Multiplexing Passive Optical Network, WDM-PON). Thecommunicating end terminals are referred to as Optical Network Unit,ONU, at the customer site and Optical Line Terminal, OLT, in a CentralOffice of the network operator.

To provide services over the access network, even in case of a failure,a resilience mechanism is often implemented. This is typically done byprotecting a part of the network with a backup network. Resilience inthe fibre optic access network is becoming increasingly important for atleast two reasons. A converged fibre optic access network needs tosupport high availability services such as Voice over IP, Businessservice, Mobile Backhaul traffic, etc. Network operators seek to achievenode consolidation, where the network switching equipment is to a higherdegree localized in centralized nodes, to save on operationalexpenditures. This means that more and more customers are served overthe access network infrastructure that connects the end user to theswitching equipment. This also means that potentially more and more endcustomers could be affected by a single failure.

A way of protecting the central parts of the access network is referredto as “dual homing”. In this scenario, a main fibre path between endusers and the central office splits into a protection fibre path thatleads to a central office in an adjacent service area.

When communication cannot be executed on the main fibre path, thecommunication is instead executed on the protection fibre path. However,the protection fibre path may not be capable of supporting the sametransmission rate as the main fibre path, thereby requiring activeequipment such as amplifiers along the fibre link in order to amplifyand restore the signal as it travels along the fibre link such that itmay be received by a receiving end terminal. However, such activeequipment is relatively costly to operate and maintain. Further, themain fibre path and the protection fibre path may not be of the samelength, thereby imposing different requirements on the main path and theprotection path.

In the case the protection fibre path is much longer than the main fibrepath, excessive requirements may be imposed on the optical transceivers.Up to 100 km transmission distances may need to be supported. Suchdistances are difficult to achieve with low cost optics. Moreover, thelink budget of access networks may be impacted by additional losses suchas connectors and impairments such as bends etc. Thus, a fibre link mayjust be slightly too long or associated with losses slightly too highfor the optical transceivers to handle. This may result in a hard stopwhere the optical transceivers simply do not get the link up forcommunication between the two end terminals, the ONU and the OLT.

SUMMARY

It is an object of the exemplifying embodiments to address at least someof the problems outlined above. In particular, it is an object of theexemplifying embodiments to provide an Optical Network Unit, ONU, anOptical Line Terminal, OLT, a method an ONU and a method in an OLT forenabling the ONU to communicate over a fibre optic access networkcomprising the ONU and the OLT. These objects and others may be obtainedby providing an ONU and an OLT and a method in an ONU and an OLTaccording to the independent claims attached below.

According to an aspect a method in an Optical Network Unit, ONU, in afibre optic access network, the fibre optic access network comprising atleast one Optical Line Terminal, OLT, and two fibre paths through thefibre optic network, a main path and a back-up path, connecting the ONUwith the OLT for enabling the ONU to communicate over the fibre opticaccess network is provided. The method comprises detecting a fault inthe main path between the ONU and the OLT, during ongoing communication.The method also comprises performing a transmission rate negotiationwith the OLT over the back-up path. When a transmission rate is agreedupon between the ONU and the OLT during the transmission ratenegotiation with regard to communication over the back-up path, themethod comprises executing communication with the OLT using the agreedtransmission rate over the back-up path.

According to an aspect, a method in an Optical Line Terminal, OLT, in afibre optic access network, the fibre optic access network comprising atleast two fibre paths between the OLT and an Optical Network Unit, ONU,a main fibre path and a back-up fibre path, for enabling the ONU tocommunicate over the fibre optic access network is provided. The methodcomprises detecting a fault in the main fibre path between the OLT andthe ONU, during ongoing communication. The method also comprisesperforming a transmission rate negotiation with the ONU over the back-uppath upon detecting the fault in the main fibre path. When atransmission rate is agreed upon between the OLT and the ONU, during thetransmission rate negotiation with regard to communication over theback-up fibre path, the method comprises executing communication withthe ONU using the agreed transmission rate over the back-up fibre path.

According to an aspect, an Optical Network Unit, ONU, in a fibre opticaccess network, the fibre optic access network comprising at least oneOptical Line Terminal, OLT, and two fibre paths through the fibre opticnetwork, a main path and a back-up path, connecting the ONU with theOLT, the ONU being adapted to enable the ONU to communicate over thefibre optic access network is provided. The ONU comprises a processingunit adapted to detect a fault in the main path between the ONU and theOLT during ongoing communication. The processing unit is also adapted toperform a transmission rate negotiation with the OLT over the back-uppath. When a transmission rate is agreed upon between the ONU and theOLT during the transmission rate negotiation with regard tocommunication over the back-up path, the processing unit is adapted toexecute communication with the OLT using the agreed transmission rateover the back-up path.

According to an aspect, an Optical Line Terminal, OLT, in a fibre opticaccess network, the fibre optic access network comprising at least twofibre paths between the OLT and an Optical Network Unit, ONU, a mainfibre path and a back-up fibre path, the OLT being adapted to enable theONU to communicate over the fibre optic access network is provided. TheOLT comprises a processing unit adapted to detect a fault in the mainfibre path between the OLT and the ONU, during ongoing communication.The processing unit is also adapted to perform a transmission ratenegotiation with the ONU over the back-up path upon detecting the faultin the main fibre path. When a transmission rate is agreed upon betweenthe OLT and the ONU, during the transmission rate negotiation withregard to communication over the back-up fibre path, the processing unitis adapted to execute communication with the ONU using the agreedtransmission rate over the back-up fibre path.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments will now be described in more detail in relation to theaccompanying drawings, in which:

FIG. 1 a is a flowchart of an exemplifying embodiment of a method in ONUfor enabling the ONU to communicate over the optic fibre access network.

FIG. 1 b is a flowchart of an exemplifying embodiment of a method in anONU for enabling the ONU to communicate over the optic fibre accessnetwork

FIG. 2 a is a flowchart of an exemplifying embodiment of a method in anOLT enabling an ONU to communicate over the fibre optic access network.

FIG. 2 b is a flowchart of an exemplifying embodiment of a method in anOLT for enabling the ONU to communicate over the optic fibre accessnetwork.

FIG. 2 c is a flowchart of an exemplifying embodiment of a method in anOLT, wherein the OLT is a back-up OLT.

FIG. 2 d is a flowchart of an exemplifying embodiment of a method in anOLT, wherein the OLT is a back-up OLT.

FIG. 3 a is a block diagram schematically illustrating an exemplifyingembodiment of an ONU and an OLT connected by two fibre link paths.

FIG. 3 b is a block diagram schematically illustrating an exemplifyingembodiment of an ONU, a main OLT connected to the ONU via a main fibrelink path, and a back-up OLT connected to the ONU via a back-up fibrelink path.

FIG. 3 c schematically illustrates several ONUs connected to an OLT viatwo fibre path links, a main fibre link and a back-up fibre link.

FIG. 3 d schematically illustrates several ONUs connected to a main OLTvia a main fibre link and connected to a back-up OLT via a back-up fibrelink.

FIG. 4 is a block diagram schematically illustrating an exemplifyingembodiment of a part of a passive optical network.

DETAILED DESCRIPTION

Briefly described, exemplifying embodiments of a method in an OpticalNetwork Unit, ONU, and a method in an Optical Line Terminal, OLT, aswell as an ONU and an OLT, are provided in a fibre optic access network,the fibre optic access network comprising at least one OLT and two fibrepaths or links through the fibre optic access network, a main fibre pathand a back-up fibre path, connecting the ONU with the at least one OLTfor enabling the ONU to communicate over the fibre optic access network.The enabling of the ONU to communicate, over the fibre optic accessnetwork, with the at least one OLT is ensured by, in case a fault in themain fibre path or link between the ONU and the OLT is detected,performing a transmission rate negotiation with the OLT over the back-uppath.

It shall be noted that the expression “fibre path” or simply “path” willbe used herein to refer to a fibre link or a fibre path link.

An exemplifying embodiment of a method in an ONU in a fibre optic accessnetwork, the fibre optic access network comprising at least one OLT andtwo fibre paths or links through the fibre optic access network, a mainfibre path and a back-up fibre path, connecting the ONU with the OLT forenabling the ONU to communicate over the fibre optic access network willnow be described with reference to FIG. 1 a. FIG. 1 a is a flowchart ofan exemplifying embodiment of a method in ONU for enabling the ONU tocommunicate over the optic fibre access network.

In this example, the method 100 in an ONU for enabling the ONU tocommunicate over the optic fibre access network comprises detecting 110a fault in the main path between the ONU and the OLT, during ongoingcommunication. The method also comprises performing 120 a transmissionrate negotiation with the OLT over the back-up path. When a transmissionrate is agreed upon between the ONU and the OLT during the transmissionrate negotiation with regard to communication over the back-up path, themethod comprises executing 150 communications with the OLT using theagreed transmission rate over the back-up path.

This exemplifying method has several advantages. By providing a back-uppath between the ONU and the at least one OLT, a cost effective way toprotect the optic fibre access network is achieved. By providing anegotiation procedure, the back-up link may be of lower “standard” andthus less expensive. By lower standard is meant that it may not becapable of supporting the same transmission rates as the main fibrelink. By providing the negotiation procedure, communication may still bepossible, but at a reduced rate. In an example the two fibre links arenot of the same length, e.g. due to running between the ONU and the OLTalong different routes or paths. Consequently, they may not be able tosupport the same transmission rates even though the fibre linksthemselves may be of the same quality, since the longer fibre path maytypically not be capable of supporting transmission rates as high as theshorter fibre path. It may be desirable to make the two fibre paths runalong different routes in order to further increase robustness. In caseone fibre link should be broken due to a cut, the other fibre cable willnot be subject to the same cut if it runs along a different path. Insuch cases, the negotiation of transmission rates enables communicationto be executed on the back-up fibre path in case the back-up fibre pathdoes not support communication at the same transmission rate as the mainfibre path.

According to an embodiment, the detecting of a fault in the main fibrepath between the ONU and the OLT comprises receiving an alarm indicatingthe fault.

In another example, the detection of a fault in the path between the ONUand the OLT comprises receiving statistical information from an FECdecoder block from which the ONU determines that a fault has occurred.

This means that in one example, the ONU receives an alarm, e.g. from theOLT and the performing 120 of a transmission rate negotiation with theOLT over the back-up path is initiated by the OLT. In another example,the ONU detects the fault in the main path between the ONU and the OLTand initiates the performing 120 of a transmission rate negotiation withthe OLT over the back-up path.

According to an embodiment, the alarm is one of “loss of signal”, “lossof synchronization”, “low signal level”, “loss of data structure” and“high bit error rate”.

An example of “loss of data structure” is loss of bytes, loss of words,loss of blocks or loss of frames. All of these alarms described abovewill indicate to the ONU a fault has occurred on the main fibre pathtoward the OLT and will trigger the ONU to perform a transmission ratenegotiation with the OLT over the back-up path.

According to still an embodiment, the method in the ONU furthercomprises negotiating 140 one or more further parameters to be used forcommunication on the back-up fibre path between the ONU and the OLT.

There are other parameters, which when used for communication on a fibrepath will affect the transmission rate or the probability of being ableto successfully communicate at a certain transmission rate withoutexcess bit error rate. Some examples of such parameters are ForwardError Correction, FEC, cryptographic capabilities and usage, modulationformat and parameters.

According to an embodiment, the transmission rate negotiation with theOLT comprises confirming use of a pre-negotiated transmission rate withregard to communication over the back-up path.

In an example, the ONU performs the transmission rate negotiation withthe OLT over the back-up path before any fault is detected in the mainfibre path. This means that in case a failure occurs in the main fibrepath, the ONU is able to quickly switch over to the back-up path. In oneexample, the transmission rate over the back-up fibre path is negotiatedbefore any session is ongoing between the ONU and the OLT over the mainfibre path and stored within the ONU. In another example, thetransmission rate is negotiated over the back-up fibre path whencommunication is ongoing over the main fibre path.

FIG. 1 b is a flowchart of an exemplifying embodiment of a method in ONUfor enabling the ONU to communicate over the optic fibre access network.

According to an exemplifying embodiment, wherein upon detecting 110 afault in the main link between the ONU and the OLT, the method furthercomprises, before performing 120 a transmission rate negotiation withthe OLT over the back-up fibre path: performing 111 a first transmissionrate negotiation with the OLT over the main path. If a transmission rateis agreed upon between the ONU and the OLT during the first transmissionrate negotiation 111, the method comprises executing 155 communicationswith the OLT using the agreed transmission rate over the main path. If atransmission rate is not agreed upon between the ONU and the OLT duringthe first transmission rate negotiation 111, the method comprisesperforming 120 a second transmission rate negotiation with the OLT overthe back-up path. When a transmission rate is agreed upon between theONU and the OLT during the second transmission rate negotiation, themethod comprises executing 150 communications with the OLT using theagreed transmission rate over the backup path.

In this embodiment, it is preferred to use the main fibre path forexecuting communication with the OLT. In such a case, the ONU firstattempts to re-establish connection with the OLT via the main fibre pathand tries to negotiate a transmission rate with the OLT over the mainfibre path. This is illustrated in FIG. 1 b by step 111. If thenegotiation with the OLT over the main fibre path is not successful,then the method continues to step 120 which is described in FIG. 1 a. Inthis exemplifying embodiment, if the negotiation is successful thencommunication is executed with the OLT over the main fibre path asindicated in step 155.

In an example, the method comprises determining the severity of thefault indicated in the alarm. For example, “loss of signal” will causethe ONU to directly initiate transmission rate negotiation with the OLTover the back-up path, whereas for example “low signal level” or “highbit error rate” will cause the ONU to first negotiate a new transmissionrate over the main fibre, which transmission rate probably will be lowerthan the currently used transmission rate, before initiatingtransmission rate negotiation with the OLT over the back-up fibre path.In this example, in case the transmission rate negotiation with the OLTover the main fibre path is successful, the ONU compares the negotiatedtransmission rate over the main fibre path to a pre-negotiatedtransmission rate over the back-up fibre path, and depending on theresult of this comparison the ONU decides to either executecommunication to the OLT over the main fibre path using the negotiatedtransmission rate or to execute communication to the OLT over theback-up fibre path using the pre-negotiated transmission rate

According to still an embodiment, the first transmission ratenegotiation 111 with the OLT over the main path results in atransmission rate which is lower than the transmission rate used beforethe detection of the fault in the main fibre path between the ONU andthe OLT. The method then further comprises comparing 114 the negotiatedtransmission rate between the ONU and the OLT over the main path withthe pre-negotiated transmission rate between the ONU and the OLT overthe backup path. If the negotiated transmission rate over the main pathis lower than the pre-negotiated transmission rate over the back-uppath, the method comprises executing 116 communications to the OLT overthe backup path using the pre-negotiated transmission rate. If thenegotiated transmission rate over the main path is higher than thepre-negotiated transmission rate over the back-up path, then the methodcomprises executing 155 communications to the OLT over the main pathusing the negotiated transmission rate.

If the first transmission rate negotiation 111 with the OLT over themain path is successful, then the negotiated transmission rate can bethe same, higher or lower than the transmission rate used before thedetection of the fault in the main fibre path. If the negotiatedtransmission rate is the same or higher than the transmission rate usedbefore the detection of the fault in the main fibre path, then thisnegotiated transmission rate is used when executing 155 communication tothe OLT over the main fibre path. This is illustrated in FIG. 1 b bystep 113, i.e. checking if the negotiated transmission rate is lowerthan the transmission rate used before the detection of the fault in themain fibre path.

If the negotiated transmission rate is lower than the transmission rateused before the detection of the fault in the main fibre path, then itmight be advantageous to execute communication over the back-up fibrepath. It may be that the negotiated transmission rate over the mainfibre path is lower than the transmission rate used before the detectionof the fault in the main fibre path but still higher than thepre-negotiated transmission rate over the back-up path.

To find this out, the method comprises comparing 114 the negotiated rateto a pre-negotiated transmission rate over the back-up path. As wasdescribed above, the transmission rate over the back-up path is in anexample negotiated beforehand with the OLT.

If the negotiated transmission rate over the main path is lower than thepre-negotiated transmission rate over the back-up path, the methodcomprises executing 116 communications to the OLT over the backup pathusing the pre-negotiated transmission rate. If, on the other hand, thenegotiated transmission rate over the main path is higher than thepre-negotiated transmission rate over the back-up fibre path, the methodcomprises executing 155 communications to the OLT over the main pathusing the negotiated transmission rate.

According to an embodiment, the ONU communicates with two physicallydistinct OLTs over the fibre optic network, a main OLT connected to theONU via the main fibre path and a back-up OLT connected to the ONU viathe back-up fibre path.

According to still an embodiment, all negotiation messages which aresent to the main OLT in the transmission rate negotiation with the ONUover the main fibre path are also sent to the back-up OLT over theback-up fibre path.

In case the ONU communicates with two separate OLTs over the fibre opticnetwork, a main OLT connected to the ONU via the main fibre path and aback-up OLT connected to the ONU via the back-up fibre path, sending alltransmission rate negotiation messages also to the back-up OLT, whichtransmission rate negotiation messages are destined to the main OLT,enables the back-up OLT to “monitor” the main OLT. In other words, theback-up OLT can deduce from the transmission negotiation messages sentfrom the ONU to the main OLT, if the transmission rate negotiation issuccessful. In case the transmission rate negotiation is not successful,the back-up OLT is in this way enabled to take action as will bedescribed below.

According to still an embodiment of the method in the ONU, thetransmission rate negotiation comprises first synchronising the ONU andthe backup OLT before the negotiation of transmission rate is performed.

An exemplifying embodiment in an Optical Line Terminal, OLT, in a fibreoptic access network, the fibre optic access network comprising at leasttwo fibre paths between the OLT and an Optical Network Unit, ONU, a mainfibre path and a back-up fibre path, for enabling the ONU to communicateover the fibre optic access network will now be described with referenceto FIG. 2 a.

FIG. 2 a is a flowchart of an exemplifying embodiment of a method in anOLT enabling an ONU to communicate over the fibre optic access network.

According to an exemplifying embodiment of the method 200 in the OLT,the method comprises detecting 210 a fault in the main fibre pathbetween the OLT and the ONU, during ongoing communication. The methodalso comprises performing 220 a transmission rate negotiation with theONU over the back-up path upon detecting the fault in the main fibrepath. When a transmission rate is agreed upon between the OLT and theONU, during the transmission rate negotiation with regard tocommunication over the back-up fibre path, the method comprisesexecuting 231 communications with the ONU using the agreed transmissionrate over the back-up fibre path.

This exemplifying method has several advantages. By providing a back-uppath between the ONU and the OLT, a cost effective way to protect theoptic fibre access network is achieved. By providing a negotiationprocedure, the back-up link may be of lower “standard” and thus lessexpensive. By lower standard is meant that it may not be capable ofsupporting the same transmission rates as the main fibre link. Byproviding the negotiation procedure, communication may still bepossible, but at a reduced rate. In an example the two fibre links arenot of the same length, e.g. due to running between the ONU and the OLTalong different routes or paths or e.g. due to being connected toseparate physical OLTs. Consequently, they may not be able to supportthe same transmission rates even though the fibre links themselves maybe of the same quality, since the longer fibre path may typically not becapable of supporting transmission rates as high as the shorter fibrepath. It may be desirable to make the two fibre paths run alongdifferent routes in order to further increase robustness. In case onefibre link should be broken due to a cut, the other fibre cable will notbe subject to the same cut if it runs along a different route. In suchcases, the negotiation of transmission rates enables communication to beexecuted on the back-up fibre path in case the back-up fibre path doesnot support communication at the same transmission rate as the mainfibre path.

According to an embodiment, the detection of a fault in the main fibrepath between the OLT and the ONU comprises receiving an alarm indicatingthe fault.

This means that in one example, the OLT receives an alarm, e.g. from theONU and the performing 220 of a transmission rate negotiation with theONU over the back-up path is initiated by the ONU. In another example,the OLT detects the fault in the main path between the OLT and the ONUand initiates the performing 220 of a transmission rate negotiation withthe OLT over the back-up path.

In another example, the detection of a fault in the path between the OLTand the ONU comprises receiving statistical information from an FECdecoder block from which the OLT determines that link performance hasdegraded to unacceptable levels.

According to an embodiment, the alarm is one of “loss of signal”, “lossof synchronization”, “low signal level”, “loss of data structure” and“high bit error rate”.

An example of “loss of data structure” is loss of bytes, loss of words,loss of blocks or loss of frames. All of these alarms described abovewill indicate to the OLT a fault has occurred on the main fibre pathtoward the ONU and will trigger the OLT to perform a transmission ratenegotiation with the ONU over the back-up path. It is understood that,in the event that main and backup OLTs are physically separateequipment, a communications link between them enables them to exchangethe necessary status and event information.

According to an embodiment, the method in the OLT further comprisesnegotiating 230 one or more further parameters to be used forcommunication on the back-up fibre path between the OLT and the ONU.

There are other parameters, which when used for communication on a fibrepath will affect the transmission rate or the probability of being ableto successfully communicate at a certain transmission rate withoutexcess bit error rate. Some examples of such parameters are ForwardError Correction, FEC, cryptographic capabilities and usage, modulationformat and parameters.

According to an embodiment, the transmission rate negotiation with theONU comprises confirming use of a pre-negotiated transmission rate withregard to communication over the back-up path.

FIG. 2 b is a flowchart of an exemplifying embodiment of a method in anOLT for enabling the ONU to communicate over the optic fibre accessnetwork.

According to an exemplifying embodiment, wherein upon detecting a faultin the main link between the OLT and the ONU, the method furthercomprises before performing 210 a first transmission rate negotiationwith the ONU over the backup path, performing 211 a first transmissionrate negotiation with the ONU over the main fibre path. If atransmission rate is agreed upon between the ONU and the OLT during thefirst transmission rate negotiation, the method comprises executing 232communications with the ONU using the agreed transmission rate over themain path. If a transmission rate is not agreed upon between the ONU andthe OLT during the first transmission rate negotiation, the methodcomprises performing 210 a second transmission rate negotiation with theONU over the back-up path. When a transmission rate is agreed uponbetween the ONU and the OLT during the second transmission ratenegotiation, the method comprises executing 231 communications with theONU using the agreed transmission rate over the backup path.

In this embodiment, it is preferred to use the main fibre path forexecuting communication with the ONU. In such a case, the OLT firstattempts to re-establish connection with the ONU via the main fibre pathand tries to negotiate a transmission rate with the ONU over the mainfibre path. This is illustrated in FIG. 2 b by step 211. If thenegotiation with the ONU over the main fibre path is not successful,then the method continues to step 220 which is described in FIG. 2 a. Inthis exemplifying embodiment, if the negotiation is successful thencommunication is executed with the ONU over the main fibre path asindicated in step 232.

According to an embodiment, the first transmission rate negotiation 211with the ONU over the main path results in a transmission rate which islower than the transmission rate used before the detection of the faultin the fibre path between the ONU and the OLT, the method furthercomprising comparing 214 the negotiated transmission rate between theONU and the OLT over the main path with the pre-negotiated transmissionrate between the ONU and the OLT over the backup path. If the negotiatedtransmission rate over the main path is lower than the pre-negotiatedtransmission rate over the back-up path, the method comprises executing216 communications to the ONU over the backup path using thepre-negotiated transmission rate. If the negotiated transmission rateover the main path is higher than the pre-negotiated transmission rateover the back-up path, the method comprises executing 232 communicationsto the ONU over the main path using the negotiated transmission rate.

If the first transmission rate negotiation 211 with the ONU over themain path is successful, then the negotiated transmission rate can bethe same, higher or lower than the transmission rate used before thedetection of the fault in the main fibre path. If the negotiatedtransmission rate is the same or higher than the transmission rate usedbefore the detection of the fault in the main fibre path, then thisnegotiated transmission rate is used when executing 232 communication tothe ONU over the main fibre path. This is illustrated in FIG. 2 b bystep 213, i.e. checking if the negotiated transmission rate is lowerthan the transmission rate used before the detection of the fault in themain fibre path

If the negotiated transmission rate is lower than the transmission rateused before the detection of the fault in the main fibre path, then itmight be advantageous to execute communication over the back-up fibrepath. It may be that the negotiated transmission rate over the mainfibre path is lower than the transmission rate used before the detectionof the fault in the main fibre path but still higher than thepre-negotiated transmission rate over the back-up path.

To find this out, the method comprises comparing 214 the negotiated rateto a pre-negotiated transmission rate over the back-up path. As wasdescribed above, the transmission rate over the back-up path is in anexample negotiated beforehand with the ONU.

If the negotiated transmission rate over the main path is lower than thepre-negotiated transmission rate over the back-up path, the methodcomprises executing 216 communications to the ONU over the backup pathusing the pre-negotiated transmission rate. If, on the other hand, thenegotiated transmission rate over the main path is higher than thepre-negotiated transmission rate over the back-up fibre path, the methodcomprises executing 232 communications to the ONU over the main pathusing the negotiated transmission rate.

According to still an embodiment, the OLT is a main OLT connected to theONU via the main fibre path and the back-up fibre path connects the ONUand a back-up OLT.

In this case, the main OLT and the back-up OLT are physically differententities. In an example, they are located in the same geographicallocation and in another example, they are located at two differentgeographical locations.

In an example of the method in a main OLT, wherein in case the back-upfibre path is to be used, the method comprises sending an instruction tothe back-up OLT, the instruction instructing the back-up OLT to initiatetransmission rate negotiation with the ONU over the back-up fibre path.

In this example, the main OLT determines, e.g. as explained above instep 216 that the communication is to be executed over the back-up fibrepath. The main OLT then sends a signal or otherwise instructs theback-up OLT that it should execute communication to the ONU over theback-up fibre path.

In still an example of the method in a main OLT, all negotiationmessages which are sent to the ONU in the transmission rate negotiationwith the ONU over the main fibre path are also sent to the back-up OLT.

In yet an example of the method in a main OLT, the method furthercomprises receiving a request from the ONU to establish communicationwith the back-up OLT, wherein the main OLT sends a request to theback-up OLT to execute communication to the ONU over the back-up fibrepath.

This situation may arise when it is the ONU that determines that theback-up fibre path is to be used to execute communication to an OLT, inthis case the back-up OLT. Then the ONU sends a request to the main OLTwhere the ONU requests the communication to be executed over the back-uppath with the back-up OLT. Then the main OLT in turn sends a request tothe back-up OLT to execute communication to the ONU over the back-upfibre path.

FIG. 2 c is a flowchart of an exemplifying embodiment of a method in anOLT, wherein the OLT is a back-up OLT.

According to an embodiment of the method in the OLT, the OLT is aback-up OLT connected to the ONU via the back-up fibre path and the mainfibre path connects the ONU and a main OLT (not shown in FIG. 2 c).

In an example of the method in a back-up OLT, the method comprisesreceiving 240 an indication that communication needs to be executed on alink between the back-up OLT and the ONU, and performing 250transmission rate negotiation with the ONU over the back-up fibre path.If the transmission rate negotiation does not result in an agreedtransmission rate to be used for executing communication between the ONUand the back-up OLT over the back-up fibre path, then the methodcomprises generating 257 a link failure message. If a transmission rateis agreed upon between the ONU and the back-up OLT, then the methodcomprises executing 256 communication with the back-up OLT using theagreed transmission rate over the back-up fibre path.

In case the transmission rate negotiation fails, it is not possible toexecute communication at any transmission rate that is supported by boththe ONU and the OLT. In such a case a link failure message is generated257.

In yet an example of the method in a back-up OLT, the method comprisesreceiving 240 the indication comprises receiving a request, from themain OLT, to initiate transmission rate negotiation with the ONU overthe back-up fibre path.

As has been described above, in case it is the ONU that determines thatthe back-up fibre path is to be used to execute communication to an OLT,in this case the back-up OLT, then the ONU sends a request to the mainOLT where the ONU requests the communication to be executed over theback-up path with the back-up OLT. Then the main OLT in turn sends arequest to the back-up OLT to execute communication to the ONU over theback-up fibre path.

FIG. 2 d is a flowchart of an exemplifying embodiment of a method in anOLT, wherein the OLT is a back-up OLT.

In yet an example of the method in a back-up OLT, wherein receiving 240the indication comprises receiving 241 transmission rate negotiationmessages from the ONU destined for the main OLT.

This means that the indication which is received 240 to establishcommunication with the ONU comprises receiving a copy of alltransmission rate negotiations messages which are sent to the main OLT.

In still an example, of the method in a back-up OLT, the method furthercomprises determining, from the received transmission rate negotiationmessages from the ONU, that an un-recoverable link failure has occurredon the main fibre path between the ONU and the main OLT.

This means that since the back-up OLT receives a copy of alltransmission rate negotiations messages which are sent to the main OLT,the back-up OLT can deduce from the received messages if thetransmission rate negotiation between the ONU and the main OLT is beingsuccessful or not.

In a further example, of the method in a back-up OLT, the transmissionrate negotiation between the back-up OLT and the ONU over the back-upfibre path comprises confirming use of a pre-negotiated transmissionrate between the ONU and the back-up OLT over the back-up fibre path

In a further example, of the method in a back-up OLT, wherein thedetermining from the received transmission rate negotiation messagesfrom the ONU or the main OLT that an un-recoverable link failure hasoccurred on the main fibre path comprises starting 242 a preset timerwhen a transmission rate negotiation message is received 241 from theONU destined for the main OLT; and if the timer expires before receivingan expected follow-up transmission rate negotiation message from the ONUdestined for the main OLT, then performing 250 transmission ratenegotiation with the ONU over the back-up path.

As transmission rate negotiation messages from the ONU destined for themain OLT are also received by the back-up OLT, the back-up OLT starts apreset timer 242 as it receives a transmission rate negotiation message.If the timer expires before any other transmission rate negotiationmessage is received then the back-OLT determines that the transmissionrate negotiation was unsuccessful. Likewise, if the back-up OLT candeduce that communication cannot be executed on the main fibre path dueto an un-recoverable link failure has occurred on the main fibre pathbetween the ONU and the main OLT, the back-up OLT performs 250transmission rate negotiation with the ONU over the back-up fibre path.

However, if a subsequent transmission negotiation message from the ONUdestined for the main OLT is received 244 by the back-up OLT or atransmission negotiation message from the main OLT destined for the ONUis received 244 by the back-up OLT before the timer expires, then theback-up OLT can determine if the main link is not up but transmissionrate negotiation is still ongoing on the main fibre path and the back-upOLT again starts 242 the preset timer.

The back-up OLT can detect 244 that communication is executed on themain fibre path between the ONU and the main OLT, e.g. by receiving aconfirmation message from either the ONU or the main OLT. If this is thecase, the method is ended 246, meaning that the back-up OLT will go backto a stand-by mode.

Exemplifying embodiments of an ONU and an OLT in a fibre optic accessnetwork, the ONU and the OLT being adapted to enable the ONU tocommunicate with the OLT will now be described with reference to FIGS. 3a-d. The ONU and the OLT have the same objects, advantages and technicalfeatures as the methods therein described above. Therefore, the ONU andthe OLT will be described briefly in order to avoid unnecessaryrepetition.

FIG. 3 a is a block diagram schematically illustrating an exemplifyingembodiment of an ONU and an OLT connected by two fibre link paths.

It will be appreciated that the OLT in an example is adapted to comprisetwo roles, a main role and a backup role, and that these roles may beimplemented either in a single OLT equipment or in two distinct OLTequipments.

In FIG. 3 a, the ONU and the OLT are illustrated having transceiverarrangements 317 and 327. These are adapted to enable the ONU 310 andthe OLT 320 to communicate with each other via a main fibre link orfibre path; or a back-up fibre link or fibre path. A transceiverarrangement may comprise one or more transmitters and one or morereceivers.

The OLT is adapted to perform a main OLT role or function and a backupOLT role or function and these roles or functions may be implementedeither in a single OLT equipment or in two distinct OLT equipments.

According to an exemplifying embodiment of the Optical Network Unit, ONU310, in a fibre optic access network, the fibre optic access networkcomprising at least one Optical Line Terminal, OLT, 320 and two fibrepaths through the fibre optic network, a main path and a back-up path,connecting the ONU 310 with the OLT 320, the ONU being adapted to enablethe ONU 310 to communicate over the fibre optic access network, the ONU310 comprises a processing unit 311 adapted to detect a fault in themain path between the ONU 310 and the OLT 320, during ongoingcommunication. The processing unit 311 is also adapted to perform atransmission rate negotiation with the OLT 320 over the back-up path.When a transmission rate is agreed upon between the ONU 310 and the OLT320 during the transmission rate negotiation with regard tocommunication over the back-up path, the processing unit 311 is adaptedto execute communication with the OLT 320 using the agreed transmissionrate over the back-up path.

According to an embodiment of the Optical Network Unit, the detection ofa fault in the main fibre path between the ONU 310 and the OLT 320comprises receiving an alarm indicating the fault.

The ONU is adapted to detect the fault, e.g. by the transceiverarrangement 317. The transceiver arrangement 317 is then adapted togenerate an alarm which is sent to the processing unit. Hence, thedetection of a fault in the main fibre path between the ONU 310 and theOLT 320 by reception of an alarm indicating the fault means that theprocessing unit 311 receives the alarm from the transceiver arrangement317.

In an example, the transceiver arrangement 317 is adapted to performcomprises signal recovery, framing alignment, error checking andcorrecting functions.

In yet an embodiment, the alarm is one of “loss of signal”, “loss ofsynchronization”, “low signal level”, “loss of data structure” and “highbit error rate”.

According to still an embodiment, the processing unit 311 is furtheradapted to negotiate one or more further parameters to be used forcommunication on the back-up fibre path between the ONU 310 and the OLT320.

According to an embodiment, the transmission rate negotiation with theOLT 320 comprises confirming use of a pre-negotiated transmission ratewith regard to communication over the back-up path.

In still an embodiment, wherein upon detecting a fault in the main linkbetween the ONU and the OLT, the processing unit 311 is further adaptedto, before performing a transmission rate negotiation with the OLT 320over the back-up fibre path, perform a first transmission ratenegotiation with the OLT 320 over the main path. If a transmission rateis agreed upon between the ONU 310 and the OLT 320 during the firsttransmission rate negotiation, the processing unit 311 is adapted toexecute communication with the OLT 320 using the agreed transmissionrate over the main path. If a transmission rate is not agreed uponbetween the ONU 310 and the OLT 320 during the first transmission ratenegotiation, the processing unit 311 is adapted to perform a secondtransmission rate negotiation with the OLT 320 over the back-up path.When a transmission rate is agreed upon between the ONU 310 and the OLT320 during the second transmission rate negotiation, the processing unit311 is adapted to execute communication with the OLT using the agreedtransmission rate over the backup path.

In yet an embodiment, wherein the first transmission rate negotiationwith the OLT 320 over the main path results in a transmission rate whichis lower than the transmission rate used before the detection of thefault in the main fibre path between the ONU 310 and the OLT 320, theprocessing unit 311 is further adapted to compare the negotiatedtransmission rate between the ONU 310 and the OLT 320 over the main pathwith the pre-negotiated transmission rate between the ONU 310 and theOLT 320 over the backup path. If the negotiated transmission rate overthe main path is lower than the pre-negotiated transmission rate overthe back-up path, then the processing unit 311 is adapted to executecommunication to the OLT 320 over the backup path using thepre-negotiated transmission rate. If the negotiated transmission rateover the main path is higher than the pre-negotiated transmission rateover the back-up path, then the processing unit 311 is adapted toexecute communication to the OLT 320 over the main path using thenegotiated transmission rate.

FIGS. 3 b and 3 d illustrate an example wherein an ONU 310 communicateswith two separate OLTs 330 and 340.

According to an embodiment, the ONU 310 is adapted to communicate withtwo separate OLTs over the fibre optic network, a main OLT 330 connectedto the ONU via the main fibre path and a back-up OLT 340 connected tothe ONU via the back-up fibre path.

As can be seen in the FIGS. 3 b and 3 d, the main OLT 330 and theback-up OLT are each connected to an ONU 310 by means of two fibrepaths, a main fibre path and a back-up fibre path. Typically, the mainOLT 330 serves a first set of ONUs and the back-up OLT serves other ONUs(not shown) for which ONUs the back-up OLT is a main OLT. Likewise themain OLT 330 may act as a back-up OLT for still other ONUs (not shown)being connected to a still further OLT (not shown).

FIGS. 3 c and 3 d also illustrate that the main fibre path and theback-up fibre path can be shared by a plurality of ONUs 310. In theembodiment illustrated in FIGS. 3 c and 3 d, various passive orsemi-passive devices, e.g. power splitters, wavelength filters and/oroptical switches, 350 and 360 could be used to enable several ONUs 310sharing the respective main and back-up fibre paths.

It shall be pointed out, that in case there should be any activeequipment in points 350 and 360, no active equipment is present betweenthe point 360 and the respective main OLT 330 or the back-up OLT 440.

In yet another example, the redundant links go all the way to the ONUs310, such that splitting nodes 350, 360 exist separately for each ofmain and backup links. In that configuration, an ONU 310 is adapted todistinguish and negotiate main and backup independently.

In yet another example, not illustrated in FIG. 3 d, there are N numberof main fibre links, one for each of the N sets of ONUs 310 where a setis either a single ONU or multiple ONUs, but just one back-up fibrerunning from the point 360 to the back-up OLT 340.

According to still an embodiment, the ONU 310 is adapted to send allnegotiation messages, which are sent to the main OLT 330 in thetransmission rate negotiation with the ONU 310 over the main fibre path,also to the back-up OLT 340 via the back-up fibre path.

In yet an embodiment, the processing unit 311 is adapted to synchronisethe ONU 310 and the OLT 320, 330, 340 before performing negotiation oftransmission rate.

According to an exemplifying embodiment of an Optical Line Terminal, OLT320, in a fibre optic access network, the fibre optic access networkcomprising at least two fibre paths between the OLT and an OpticalNetwork Unit, ONU, (310) a main fibre path and a back-up fibre path, theOLT being adapted to enable the ONU to communicate over the fibre opticaccess network, the OLT comprises a processing unit 321 adapted todetect a fault in the main fibre path between the OLT 320 and the ONU310, during ongoing communication. The processing unit 321 is alsoadapted to perform a transmission rate negotiation with the ONU 310 overthe back-up path upon detecting the fault in the main fibre path. When atransmission rate is agreed upon between the OLT 320 and the ONU 310,during the transmission rate negotiation with regard to communicationover the back-up fibre path, the processing unit 321 is adapted toexecute communication with the ONU 310 using the agreed transmissionrate over the back-up fibre path.

According to an embodiment of the OLT 320, the detection of a fault inthe main fibre path between the OLT 320 and the ONU 310 comprisesreceiving an alarm indicating the fault.

According to still an embodiment, the alarm is one of “loss of signal”,“loss of synchronization”, “low signal level”, “loss of data structure”and “high bit error rate”.

In yet an embodiment, the processing unit 321 is further adapted tonegotiate one or more further parameters to be used for communication onthe back-up fibre path between the OLT 320 and the ONU 310.

In a further embodiment, the transmission rate negotiation with the ONU310 comprises confirming use of a pre-negotiated transmission rate withregard to communication over the back-up path.

According to an embodiment, wherein upon detecting a fault in the mainlink between the ONU 310 and the OLT 320, the processing unit 321 isfurther adapted to, before performing a first transmission ratenegotiation with the OLT 320 over the back-up path, perform a firsttransmission rate negotiation with the ONU 310 over the main fibre path.If a transmission rate is agreed upon between the ONU 310 and the OLT320 during the first transmission rate negotiation, the processing unit321 is adapted to execute communication with the ONU 310 using theagreed transmission rate over the main path. If a transmission rate isnot agreed upon between the ONU 310 and the OLT 320 during the firsttransmission rate negotiation, the processing unit 321 is adapted toperform a second transmission rate negotiation with the ONU 310 over theback-up path. When a transmission rate is agreed upon between the ONU310 and the OLT 320 during the second transmission rate negotiation, theprocessing unit 321 is adapted to execute communication with the ONU 310using the agreed transmission rate over the backup path.

According to still an embodiment, of the OLT 320, the first transmissionrate negotiation with the ONU 310 over the main path results in atransmission rate which is lower than the transmission rate used beforethe detection of the fault in the link between the ONU 310 and the OLT320, the processing unit 321 is further adapted to compare thenegotiated transmission rate between the ONU 310 and the OLT 320 overthe main path with the pre-negotiated transmission rate between the ONU310 and the OLT 320 over the backup path. If the negotiated transmissionrate over the main path is lower than the pre-negotiated transmissionrate over the back-up path, then the processing unit 321 is adapted toexecute communication to the ONU 310 over the backup path using thepre-negotiated transmission rate. If the negotiated transmission rateover the main path is higher than the pre-negotiated transmission rateover the back-up path, then the processing unit 321 is adapted toexecute communication to the ONU 310 over the main path using thenegotiated transmission rate.

In still a further embodiment, the OLT 330 is a main OLT 330 connectedto the ONU 310 via the main fibre path and the back-up fibre pathconnects the ONU 310 and a back-up OLT 340.

In this case, the main OLT and the back-up OLT are physically differententities. In an example, they are located in the same geographicallocation and in another example, they are located at two differentgeographical locations.

In an example of the main OLT 330, wherein in case the back-up fibrepath is to be used, the processing unit 331 is adapted to send aninstruction to the back-up OLT 340, the instruction instructing theback-up OLT 340 to initiate transmission rate negotiation with the ONU310 over the back-up fibre path.

In yet an example of the main OLT 330, the processing unit 331 isadapted to send all negotiation messages, which are sent to the ONU 310in the transmission rate negotiation with the ONU 310 over the mainfibre path, also to the back-up OLT 340 over the back-up fibre path.

In still an example of the main OLT 330, the processing unit 331 isadapted to receive an alarm indicating a severe failure, whereincommunication will not be able to be restored on the main fibre path atany transmission rate. Then the processing unit 331 is adapted to orderto the back-up OLT 340 to initiate transmission rate negotiation withthe ONU over the back-up path.

In still a further example of the main OLT 330, the processing unit 331is further adapted to receive a request from the ONU 310 to establishcommunication with the back-up OLT 340, wherein the processing unit 331is adapted to send a request to the back-up OLT 340 to executecommunication to the ONU 310 over the back-up fibre path.

According to an embodiment, the OLT 340 is a back-up OLT 340 connectedto the ONU 310 via the back-up fibre path and the main fibre pathconnects the ONU 310 and a main OLT 330.

In an example of the back-up OLT 340, the processing unit 341 is furtheradapted to receive an indication that communication needs to be executedon a link between the back-up OLT 340 and the ONU (310), and to performtransmission rate negotiation with the ONU 310 over the back-up fibrepath. If the transmission rate negotiation does not results in an agreedtransmission rate to be used for executing communication between the ONU310 and the back-up OLT 340 over the back-up fibre path, then theprocessing unit 341 is adapted to generate a link failure message. If atransmission rate is agreed upon between the ONU 310 and the back-up OLT340, then the processing unit 341 is adapted to execute communicationwith the back-up OLT 340 using the agreed transmission rate over theback-up fibre path.

In still an example of the back-up OLT 340, the reception of theindication comprises receiving a request, from the main OLT 330, toinitiate transmission rate negotiation with the ONU 310 over the back-upfibre path.

In still a further example of the back-up OLT 340, the reception of theindication comprises receiving transmission rate negotiation messagesfrom the ONU 310 destined for the main OLT 330.

In yet an example of the back-up OLT 340, the processing unit 341 isfurther adapted to determine, from the received transmission ratenegotiation messages from the ONU 310, that a un-recoverable linkfailure has occurred on the main fibre path between the ONU 310 and themain OLT 330.

In a further example of the back-up OLT 340, the transmission ratenegotiation between the back-up OLT 340 and the ONU 310 comprises theprocessing unit 341 being adapted to confirm use of pre-negotiatedtransmission rate between the ONU 310 and the back-up OLT 340 over theback-up fibre path.

In yet an example of the back-up OLT 340, the processing unit 341 isadapted to, when determining from the received transmission ratenegotiation messages from the ONU 310 or the main OLT 330 that aun-recoverable link failure has occurred on the main fibre path, start apreset timer when a transmission rate negotiation message is receivedfrom the ONU 310 destined for the main OLT 330. If the timer expiresbefore a receiving an expected follow-up transmission rate negotiationmessage from the ONU 310 destined for the main OLT 330, then theprocessing unit 341 is adapted to perform transmission rate negotiationwith the ONU 310 over the back-up path.

In the case that the main OLT and the back-up OLT are geographicallyco-located, the OLT in one example does not duplicate the fullfunctionality of the main OLT. In one example of such an OLT, itcomprises only a transceiver arrangement and the back-up OLT isconnected to the main OLT, which is located at the same geographicallocation, via a separate link.

The exemplifying embodiments of the ONU, the OLT and the respectivemethods therein provides a protection mechanism for the access networkthat doesn't require active equipment between the end user and theback-up OLT over the protection fibre path. Typically, the OLT islocated in a Central Office, CO, of the network operator. It is costeffective since a part of the capacity over a link can be protected,avoiding the potential high costs of providing protection of the fulllink capacity. The exemplifying embodiments enable a damaged link to berestored, e.g. the main fibre path/link may still be used even though itis damaged by lowering the transmission rate. This way, the main fibrepath may still be capable of supporting communication between the ONUand the OLT. Alternatively, if the damaged main fibre path is notcapable of supporting communication between the ONU and the OLT, thenthe back-up fibre path is used to support communication between the ONUand the OLT.

According to an embodiment, when a fault has occurred on the main fibrepath resulting in either lowering the transmission rate on the mainfibre path or switching to the back-up fibre path, the OLT is adapted toprioritise traffic and selectively discard traffic which exceeds thecapacity of the fibre path used for communication. In an example, thisis done in a manner such that service level agreements, SLAs, areQuality-of-Service, QoS, requirements or guarantees are fulfilled.

FIG. 4 is a block diagram schematically illustrating an exemplifyingembodiment of a part of a passive optical network.

FIG. 4 illustrates two exemplifying configurations enabling an ONU 419to communicate over a main fibre path and a back-up fibre path. Theupper configuration illustrates N number of ONUs 410 being connected toa 2:N multiplexer 470. The multiplexer 470 is e.g. a power splitter, awavelength splitter or an Arrayed Waveguide Grating AWG. In case an AWGis used as the multiplexer, an extra port on the AWG is provided toconnection of the back-up fibre path. If the multiplexer 470 is a 2:NAWG, the user wavelength will appear differently at the back-up OLTcompared to the main OLT; wavelength 1 in the main OLT will correspondto wavelength 2 in the back-up OLT.

The lower configuration illustrates an ONU 410 being connected to anoptical 1:2 splitter 480.

In the described embodiments, forward error correction (FEC) mayoptionally be used. While finding the maximum transmission rate overspecific link, FEC can help to give additional link budget. An exampleof such code is the Reed-Solomon (255,239) code which gives a bandwidthoverhead of roughly 7% and increase the link budget with roughly 3-5 dB.There are multiple FEC codes that can be used. For example, twodifferent FEC codes could be supported; one referred to as strong FEC(giving higher link budget increase at the cost of higher bandwidthoverhead), and one referred to as weak FEC (giving lower link budgetincrease at the cost of lower bandwidth overhead). When the endterminals negotiate which transmission rate to use over a link,according to an embodiment, several other parameters may be negotiated,both with regard to communication to the other end terminal and alsowith regard to communication received from the other end terminal.

The ONU, 310 the OLT 320, the main OLT 330 and the back-up OLT 340 areillustrated in FIGS. 3 a and 3 b to comprise a respective processingunit 311, 321, 331 and 341 which in turn are illustrated comprisingdedicated modules adapted to perform different method steps, e.g.synchronising module, negotiation module, timing module andcommunication module. It shall be noted that this is merely anexemplifying illustration of the respective processing unit 311, 321,331 and 341. Further FIGS. 3 a and 3 b illustrate the ONU, 310 the OLT320, the main OLT 330 and the back-up OLT 340 comprising a memory 312,322, 332 and 342. Further, the, 310 the OLT 320, the main OLT 330 andthe back-up OLT 340 are illustrated having transceiver arrangements 317,327, 337 and 347 adapted to enable the ONU 310 to communicate with theother terminals via a main fibre link or fibre path; or a back-up fibrelink or fibre path. A transceiver arrangement may comprise one or moretransmitters and one or more receivers. Further, FIGS. 3 b and 3 dillustrate the main OLT and the back-up OLT being connected by aseparate link or path. This ensures that the main OLT and the back-upOLT are able to communicate with each other without making use of themain fibre path/link or the back-up fibre path/link.

It shall be noted that the main fibre path and the back-up fibre pathmay run all the way between the ONU and the OLT, main OLT or back-upOLT. Alternatively, the main fibre path and the back-up fibre path mayrun only along a part of the way between the ONU and the OLT, main OLTor back-up OLT. Further, in case there is just one OLT adapted tofunction as a main OLT and a back-up OLT, or in case there are a mainOLT and a back-up OLT located at the same geographical location, themain fibre path and the back-up fibre path are in one example runningalong the same route and in another example running along differentroutes. Further, the fibre topology can be a tree, a ring or a mixtureof the two.

It should be noted that FIGS. 3 a and 3 b merely illustrates variousfunctional modules and units in the ONU, 310 the OLT 320, the main OLT330 and the back-up OLT 340 in a logical sense. The functions inpractice may be implemented using any suitable software and hardwaremeans/circuits etc. Thus, the embodiments are generally not limited tothe shown structures of the ONU, 310 the OLT 320, the main OLT 330 andthe back-up OLT 340 and the functional modules and units. Hence, thepreviously described exemplary embodiments may be realised in many ways.For example, one embodiment includes a computer-readable medium havinginstructions stored thereon that are executable by the respectiveprocessing unit 311, 321, 331 and 341 for performing the method steps ofthe exemplifying embodiments as set forth in the claims. Theinstructions executable by the computing system and stored on thecomputer-readable medium perform the method steps of the exemplifyingembodiments as set forth in the claims.

While the embodiments have been described in terms of severalembodiments, it is contemplated that alternatives, modifications,permutations and equivalents thereof will become apparent upon readingof the specifications and study of the drawings. It is thereforeintended that the following appended claims include such alternatives,modifications, permutations and equivalents as fall within the scope ofthe embodiments and defined by the pending claims.

1-56. (canceled)
 57. A method in an Optical Network Unit (ONU) in afiber optic access network, the fiber optic access network comprising atleast one Optical Line Terminal (OLT) and two fiber paths through thefiber optic access network, a main fiber path and a back-up path,connecting said ONU with said OLT for enabling the ONU to communicateover the fiber optic access network, the method comprising: detecting afault in the main fiber path between said ONU and said OLT, duringongoing communication; performing a transmission rate negotiation withsaid OLT over the back-up path; and when a transmission rate is agreedupon between said ONU and said OLT during the transmission ratenegotiation with regard to communication over the back-up path,executing communication with said OLT using the agreed upon transmissionrate over the back-up path.
 58. The method according to claim 57,wherein detecting the fault in the main fiber path between said ONU andsaid OLT comprises receiving an alarm indicating the fault.
 59. Themethod according to claim 58, wherein the alarm comprises one of a “lossof signal” alarm, a “loss of synchronization” alarm, a “low signallevel” alarm, a “loss of data structure” alarm, and a “high bit errorrate” alarm.
 60. The method according to claim 57, further comprisingnegotiating one or more further parameters to be used for communicationon the back-up path between said ONU and said OLT.
 61. The methodaccording to claim 57, wherein performing the transmission ratenegotiation with said OLT comprises confirming use of a pre-negotiatedtransmission rate with regard to communication over the back-up path.62. The method according to claim 57, wherein upon detecting the faultin the main fiber path between said ONU and said OLT, the method furthercomprises, before performing the transmission rate negotiation with saidOLT over the back-up path: performing a first transmission ratenegotiation with said OLT over the main fiber path; if a firsttransmission rate is agreed upon between said ONU and said OLT duringthe first transmission rate negotiation, executing communication withsaid OLT using the first transmission rate over the main fiber path; ifthe first transmission rate is not agreed upon between said ONU and saidOLT during the first transmission rate negotiation, performing a secondtransmission rate negotiation with said OLT over the back-up path; andwhen a second transmission rate is agreed upon between said ONU and saidOLT during the second transmission rate negotiation, executingcommunication with said OLT using the second transmission rate over theback-up path.
 63. The method according to claim 62, wherein performingthe first transmission rate negotiation with said OLT over the mainfiber path results in the first transmission rate, which is lower thanthe transmission rate used before the detection of the fault in the mainfiber path between said ONU and said OLT, the method further comprising:comparing the first transmission rate between said ONU and said OLT overthe main fiber path with a pre-negotiated transmission rate between saidONU and said OLT over the back-up path; if the first transmission rateover the main fiber path is lower than the pre-negotiated transmissionrate over the back-up path, executing communication to said OLT over theback-up path using the pre-negotiated transmission rate; and if thefirst transmission rate over the main fiber path is higher than thepre-negotiated transmission rate over the back-up path, executingcommunication to said OLT over the main fiber path using the firsttransmission rate.
 64. The method according to claim 57, wherein the ONUcommunicates with two separate OLTs over the fiber optic access network:a main OLT connected to the ONU via the main fiber path and a back-upOLT connected to the ONU via the back-up fiber path.
 65. The methodaccording to claim 64, further comprising wherein all negotiationmessages sent to said main OLT during the transmission rate negotiationwith said ONU over the main fiber path are also sent to the back-up OLT.66. The method according to claim 57, further comprising firstsynchronizing said ONU and said OLT before performing the transmissionrate negotiation.
 67. A method in an Optical Line Terminal (OLT) in afiber optic access network, the fiber optic access network comprising atleast two fiber paths between the OLT and an Optical Network Unit (ONU),a main fiber path and a back-up fiber path, for enabling the ONU tocommunicate over the fiber optic access network, the method comprising:detecting a fault in the main fiber path between said OLT and said ONUduring ongoing communication; performing a transmission rate negotiationwith said ONU over the back-up path upon detecting the fault in the mainfiber path; when a transmission rate is agreed upon between said OLT andsaid ONU during the transmission rate negotiation with regard tocommunication over the back-up fiber path, executing communication withsaid ONU using the agreed upon transmission rate over the back-up fiberpath.
 68. The method according to claim 67, wherein detecting the faultin the main fiber path between said OLT and said ONU comprises receivingan alarm indicating the fault.
 69. The method according to claim 68,wherein the alarm comprises one of a “loss of signal” alarm, a “loss ofsynchronization” alarm, a low signal level” alarm, a “loss of datastructure” alarm, and a “high bit error rate” alarm.
 70. The methodaccording to claim 67, further comprising negotiating one or morefurther parameters to be used for communication on the back-up fiberpath between said OLT and said ONU.
 71. The method according to claim67, wherein performing the transmission rate negotiation with said ONUcomprises confirming use of a pre-negotiated transmission rate withregard to communication over the back-up path.
 72. The method accordingto claim 67, wherein upon detecting the fault in the main link betweensaid ONU and said OLT, the method further comprises, before performingthe transmission rate negotiation with the OLT over the back-up path:performing a first transmission rate negotiation with said ONU over themain fiber path; if a first transmission rate is agreed upon betweensaid ONU and said OLT during the first transmission rate negotiation,executing communication with said ONU using the first transmission rateover the main fiber path; if the first transmission rate is not agreedupon between said ONU and said OLT during the first transmission ratenegotiation, performing a second transmission rate negotiation with saidONU over the back-up fiber path; and when a second transmission rate isagreed upon between said ONU and said OLT during the second transmissionrate negotiation, executing communication with said ONU using the secondtransmission rate over the back-up fiber path.
 73. The method accordingto claim 72, wherein performing the first transmission rate negotiationwith said ONU over the main fiber path results in the first transmissionrate, which is lower than the transmission rate used before thedetection of the fault in the main fiber path between said ONU and saidOLT, the method further comprising: comparing the first transmissionrate between said ONU and said OLT over the main fiber path with apre-negotiated transmission rate between said ONU and said OLT over theback-up fiber path; if the first transmission rate over the main fiberpath is lower than the pre-negotiated transmission rate over the back-upfiber path, executing communication to said ONU over the back-up fiberpath using the pre-negotiated transmission rate; and if the firsttransmission rate over the main fiber path is higher than thepre-negotiated transmission rate over the back-up fiber path, executingcommunication to said ONU over the main fiber path using the firsttransmission rate.
 74. The method according to claim 67, wherein saidOLT comprises a main OLT connected to said ONU via the main fiber path,and wherein the back-up fiber path connects said ONU to a back-up OLT.75. The method according to claim 74, wherein when the back-up fiberpath is to be used, the method comprises sending an instruction to theback-up OLT instructing the back-up OLT to initiate the transmissionrate negotiation with said ONU over the back-up fiber path.
 76. Themethod according to claim 74, wherein all negotiation messages sent tosaid ONU in during the transmission rate negotiation with said ONU overthe main fiber path are also sent to the back-up OLT.
 77. The methodaccording to claim 74, further comprising receiving a request from saidONU to establish communication with the back-up OLT, wherein the mainOLT sends a request to the back-up OLT to execute communication to saidONU over the back-up fiber path.
 78. The method according to claim 67,wherein the OLT comprises a back-up OLT connected to said ONU via theback-up fiber path and the main fiber path connects said ONU to a mainOLT.
 79. The method according to claim 78, further comprising: receivingan indication that communication needs to be executed on a link betweensaid back-up OLT and said ONU; performing the transmission ratenegotiation between said back-up OLT and said ONU over the back-up fiberpath; if the transmission rate negotiation does not result in the agreedupon transmission rate to be used for executing communication betweensaid ONU and said back-up OLT over the back-up fiber path, generating alink failure message; and if the transmission rate is agreed uponbetween said ONU and said back-up OLT, executing communication with theback-up OLT using the agreed upon transmission rate over the back-upfiber path.
 80. The method according to claim 79, wherein receiving theindication comprises receiving a request, from said main OLT, toinitiate the transmission rate negotiation with said ONU over theback-up fiber path.
 81. The method according to claim 79, whereinreceiving the indication comprises receiving transmission ratenegotiation messages from said ONU destined for said main OLT.
 82. Themethod according to claim 79, wherein performing the transmission ratenegotiation between said back-up OLT and said ONU over the back-up fiberpath comprises confirming use of a pre-negotiated transmission ratebetween said ONU and said back-up OLT over the back-up fiber path. 83.The method according to claim 81, further comprising determining, fromthe transmission rate negotiation messages received from said ONU, thatan un-recoverable link failure has occurred on the main fiber pathbetween said ONU and said main OLT.
 84. The method according to claim83, wherein determining that the un-recoverable link failure hasoccurred on the main fiber comprises: starting a preset timer when oneof the transmission rate negotiation messages is received from said ONUdestined for said main OLT; and if the preset timer expires before areceiving an expected follow-up transmission rate negotiation messagefrom said ONU destined for said main OLT, performing the transmissionrate negotiation with said ONU over the back-up fiber path.
 85. AnOptical Network Unit (ONU) in a fiber optic access network, the fiberoptic access network comprising at least one Optical Line Terminal (OLT)and two fiber paths through the fiber optic access network, a main fiberpath and a back-up fiber path, connecting said ONU with said OLT, theONU configured to enable the ONU to communicate over the fiber opticaccess network, the ONU comprising a processing circuit configured to:detect a fault in the main fiber path between said ONU and said OLTduring ongoing communication; perform a transmission rate negotiationwith said OLT over the back-up fiber path; and when a transmission rateis agreed upon between said ONU and said OLT during the transmissionrate negotiation with regard to communication over the back-up fiberpath, execute communication with said OLT using the agreed upontransmission rate over the back-up fiber path.
 86. The ONU according toclaim 85, wherein the processing circuit detects the fault in the mainfiber path between said ONU and said OLT by receiving an alarmindicating the fault.
 87. The ONU according to claim 86, wherein thealarm comprises one of a “loss of signal” alarm, a “loss ofsynchronization” alarm, a low signal level” alarm, a “loss of datastructure” alarm, and a “high bit error rate” alarm.
 88. The ONUaccording to claim 85, wherein the processing circuit is furtherconfigured to negotiate one or more further parameters to be used forcommunication on the back-up fiber path between said ONU and said OLT.89. The ONU according to claim 85, wherein the processing circuitperforms the transmission rate negotiation with said OLT by confirminguse of a pre-negotiated transmission rate with regard to communicationover the back-up fiber path.
 90. The ONU according to claim 85, whereinthe processing circuit, upon detecting the fault in the main fiber pathbetween said ONU and said OLT, is further configured to, beforeperforming the transmission rate negotiation with said OLT over theback-up fiber path: perform a first transmission rate negotiation withsaid OLT over the main fiber path; if a first transmission rate isagreed upon between said ONU and said OLT during the first transmissionrate negotiation, execute communication with said OLT using the firsttransmission rate over the main fiber path; if the first transmissionrate is not agreed upon between said ONU and said OLT during the firsttransmission rate negotiation, perform a second transmission ratenegotiation with said OLT over the back-up fiber path; and when a secondtransmission rate is agreed upon between said ONU and said OLT duringthe second transmission rate negotiation, execute communication withsaid OLT using the second transmission rate over the back-up fiber path.91. The ONU according to claim 90, wherein the first transmission ratenegotiation with said OLT over the main fiber path results in the firsttransmission rate, which is lower than the transmission rate used beforethe detection of the fault in the main fiber path between said ONU andsaid OLT, and wherein the processing circuit is further configured to:compare the first transmission rate between said ONU and said OLT overthe main fiber path with a pre-negotiated transmission rate between saidONU and said OLT over the back-up fiber path; if the first transmissionrate over the main fiber path is lower than the pre-negotiatedtransmission rate over the back-up fiber path, the processing circuit isconfigured to execute communication to said OLT over the back-up fiberpath using the pre-negotiated transmission rate; and if the firsttransmission rate over the main fiber path is higher than thepre-negotiated transmission rate over the back-up fiber path, theprocessing circuit is configured to execute communication to said OLTover the main fiber path using the first transmission rate.
 92. The ONUaccording to claim 85, wherein said ONU is configured to communicatewith two separate OLTs over the fiber optic access network, a main OLTconnected to said ONU via the main fiber path and a back-up OLTconnected to said ONU via the back-up fiber path.
 93. The ONU accordingto claim 92, wherein said ONU is configured to send all negotiationmessages, which are sent to said main OLT during the transmission ratenegotiation with said ONU over the main fiber path, also to said back-upOLT via the back-up fiber path.
 94. The ONU according to claim 85,wherein the processing circuit is configured to synchronize said ONU andsaid OLT before performing the negotiation of the transmission rate. 95.An Optical Line Terminal (OLT) in a fiber optic access network, thefiber optic access network comprising at least two fiber paths betweenthe OLT and an Optical Network Unit, ONU, a main fiber path and aback-up fiber path, the OLT being configured to enable the ONU tocommunicate over the fiber optic access network, the OLT comprising aprocessing circuit configured to: detect a fault in the main fiber pathbetween said OLT and said ONU during ongoing communication; perform atransmission rate negotiation with said ONU over the back-up fiber pathupon detecting the fault in the main fiber path; and when a transmissionrate is agreed upon between said OLT and said ONU during thetransmission rate negotiation with regard to communication over theback-up fiber path, the processing circuit is configured to executecommunication with said ONU using the agreed upon transmission rate overthe back-up fiber path.
 96. The OLT according to claim 95, wherein theprocessing circuit detects the fault in the main fiber path between saidOLT and said ONU by receiving an alarm indicating the fault.
 97. The OLTaccording to claim 96, wherein the alarm comprises one of a “loss ofsignal” alarm, a “loss of synchronization” alarm, a low signal level”alarm, a “loss of data structure” alarm, and a “high bit error rate”alarm.
 98. The OLT according to claim 95, wherein the processing circuitis further configured to negotiate one or more further parameters to beused for communication on the back-up fiber path between said OLT andsaid ONU.
 99. The OLT according to claim 95, wherein the processingcircuit performs the transmission rate negotiation with said ONU byconfirming use of a pre-negotiated transmission rate with regard tocommunication over the back-up fiber path.
 100. The OLT according toclaim 95, wherein upon detecting the fault in the main link between saidONU and said OLT, the processing circuit is further configured to,before performing the transmission rate negotiation with said OLT overthe back-up fiber path: perform a first transmission rate negotiationwith said ONU over the main fiber path; if a first transmission rate isagreed upon between said ONU and said OLT during the first transmissionrate negotiation, the processing circuit is configured to executecommunication with said ONU using the first transmission rate over themain fiber path; if the first transmission rate is not agreed uponbetween said ONU and said OLT during the first transmission ratenegotiation, the processing circuit is configured to perform a secondtransmission rate negotiation with said ONU over the back-up fiber path;and when a second transmission rate is agreed upon between said ONU andsaid OLT during the second transmission rate negotiation, the processingcircuit is configured to execute communication with said ONU using thesecond transmission rate over the back-up fiber path.
 101. The OLTaccording to claim 100, wherein the first transmission rate negotiationwith said ONU over the main fiber path results in the first transmissionrate, which is lower than the transmission rate used before thedetection of the fault in the main fiber path between said ONU and saidOLT, and wherein the processing circuit is further configured to:compare the first transmission rate between said ONU and said OLT overthe main fiber path with a pre-negotiated transmission rate between saidONU and said OLT over the back-up fiber path; if the first transmissionrate over the main fiber path is lower than the pre-negotiatedtransmission rate over the back-up fiber path, then the processingcircuit is configured to execute communication to said ONU over theback-up fiber path using the pre-negotiated transmission rate; and ifthe first transmission rate over the main fiber path is higher than thepre-negotiated transmission rate over the back-up fiber path, then theprocessing circuit is configured to execute communication to said ONUover the main fiber path using the first transmission rate.
 102. The OLTaccording to claim 95, wherein said OLT comprises a main OLT connectedto said ONU via the main fiber path and the back-up fiber path connectssaid ONU to a back-up OLT.
 103. The OLT according to claim 102, whereinin case the back-up fiber path is to be used, the processing circuit isconfigured to send an instruction to said back-up OLT instructing saidback-up OLT to initiate the transmission rate negotiation with said ONUover the back-up fiber path.
 104. The OLT according to claim 102,wherein the processing circuit is configured to send all negotiationmessages, which are sent to said ONU during the transmission ratenegotiation with said ONU over the main fiber path, also to said back-upOLT over the back-up fiber path.
 105. The OLT according to claim 102,wherein the processing circuit is further configured to receive arequest from said ONU to establish communication with said back-up OLT,wherein the processing circuit is configured to send a request to saidback-up OLT to execute communication to said ONU over the back-up fiberpath.
 106. The OLT according to claim 95, wherein said OLT comprises aback-up OLT connected to said ONU via the back-up fiber path, andwherein the main fiber path connects said ONU to a main OLT.
 107. TheOLT according to claim 106, wherein the processing circuit is furtherconfigured to: receive an indication that communication needs to beexecuted on a link between said back-up OLT and said ONU; perform thetransmission rate negotiation with said ONU over the back-up fiber path;if the transmission rate negotiation does not results in thetransmission rate to be used for executing communication between saidONU and said back-up OLT over the back-up fiber path, generate a linkfailure message; and if the transmission rate is agreed upon betweensaid ONU and said back-up OLT, then execute communication with saidback-up OLT using the agreed upon transmission rate over the back-upfiber path.
 108. The back-up OLT according to claim 107, wherein theprocessing circuit receives the indication by receiving a request, fromsaid main OLT, to initiate the transmission rate negotiation with saidONU over the back-up fiber path.
 109. The OLT according to claim 107,wherein the processing circuit receives the indication by receivingtransmission rate negotiation messages from said ONU destined for saidmain OLT.
 110. The OLT according to claim 107, wherein the processingcircuit performs the transmission rate negotiation between said back-upOLT and said ONU by confirming use of a pre-negotiated transmission ratebetween said ONU and said back-up OLT over the back-up fiber path. 111.The OLT according to claim 109, wherein the processing circuit isfurther configured to determine, from the received transmission ratenegotiation messages, that an un-recoverable link failure has occurredon the main fiber path between said ONU and said main OLT.
 112. The OLTaccording to claim 111, wherein the processing circuit is configured to,when determining from the received transmission rate negotiationmessages that an un-recoverable link failure has occurred on the mainfiber path, start a preset timer when one of the transmission ratenegotiation messages is received from said ONU destined for said mainOLT; and if the timer expires before the processing circuit receives anexpected follow-up transmission rate negotiation message from said ONUdestined for said main OLT, then the processing circuit is furtherconfigured to perform the transmission rate negotiation with said ONUover the back-up fiber path.